Spencer Creek Bridge Replacement and Highway 101 Realignment
While replacing the Spencer Creek Bridge, the Oregon Department of Transportation (ODOT) also realigned vulnerable sections of Highway 101 approaching the bridge, shifting the highway 50 feet inland in order to avoid expected sea cliff erosion impacts over the intended design life of the bridge and highway.
The original sections of Highway 101 to the north and south of the Spencer Creek Bridge project were vulnerable to sea cliff slope instability and landslide hazards. Aerial photograph evidence suggests that sections of the sea cliff west of Highway 101 had eroded as much as 50 to 60 feet in the 50 to 60 years prior to the bridge replacement project.
Technical studies performed by the U.S. Army Corps of Engineers (Corps) concluded that landslides occurred in the sea cliff west of and beneath the original roadway due to a combination of several factors: layers of weak, porous sediment in the sea cliff; stronger rock layers tilted at a steep angle; groundwater saturation of porous layers, especially during winter storms; ocean wave erosion removing rock from the base of the sea cliff; and surface erosion from rainwater. The Corps’ studies indicated that, in the most severely erosive areas, erosion of the sea cliff was occurring at approximately one foot per year on average over the previous 30 years. The results of the Corps’ study informed the environmental impact assessment process for the Spencer Creek Bridge replacement, as any design alternatives would either require realigning the highway away from the sea cliff or armoring to prevent the cliff erosion predicted by the study.
The Spencer Creek Bridge was closed to traffic in 1999, and it was approved for replacement in the Statewide Transportation Improvement Program (STIP) for 2002-2005. The first phase of design review evaluated nine roadway alignments, five cliff stabilization options, and nine shoreline erosion protection options. Of all of the early alternatives, two alternatives were advanced for further review in the Draft Environmental Impact Statement (DEIS); both of these alternatives replaced the bridge and realigned the highway inland away from the sea cliff, which would protect the highway from erosion while not requiring sea cliff stabilization measures or shoreline erosion protection. Alternative F would realign the highway 50 feet east based on the 50-year erosion line projection, while Alternative G would substantially reroute the highway inland away from the cliff, but through the community of Beverly Beach. ODOT selected Alternative F in the Final Environmental Impact Statement (FEIS), after balancing the extensive costs of rerouting the highway and the impact on the community with the need to avoid erosion hazards. ODOT also considered several other restrictions on Highway 101: the highway is a designated Priority 1 Lifeline Route for emergency response and evacuation, as well as a National Scenic Byway to provide public access to Oregon’s coastal resources.
The Spencer Creek Bridge replacement is included in ODOT’s Climate Adaptation Strategy as an example of the agency’s adaptive capacity. By electing to shift the highway alignment east based on the projected future sea cliff erosion impacts, ODOT hopes to protect the highway from erosion damage over the intended design life of the project while minimizing the necessity of armoring and sea cliff stabilization.
This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on October 30, 2015.
Publication Date: May 9, 2006
- Oregon Department of Transportation (ODOT)
- Best practice
- Case study